1. Field of the Invention
The present invention relates to a control system for controlling a working apparatus, particularly a change speed apparatus, for use on a bicycle, which control system is reciprocally operable by a pulling force and a restoring force.
2. Description of the Related Art
As a known type of change speed apparatus for a bicycle, a self-contained change speed apparatus is disclosed in U.S. Pat. No. 5,273,500, for example. This change speed apparatus has a fixed shaft rotatably supporting a drive member and a hub body, and a drive transmitting mechanism for transmitting torque from the drive member to the hub body in varied speeds. The drive transmitting mechanism includes four sun gears rotatably mounted on the fixed shaft, a pair of planet gears meshed with each sun gear, a clutch mechanism for transmitting torque from the drive member to a carder of one pair of planet gears, transmission pawls for transmitting torque from a ring gear meshed with the other pair of planet gears to the hub body, and transmission pawls for transmitting torque from a carder supporting the other pair of planet gears to the hub body.
The fixed shaft further supports a shifter sleeve mounted thereon to be rotatable in opposite directions. The shifter sleeve is rotatable in one direction by a pulling force exerted through a control cable, and in the other direction by a force of a return spring. The shifter sleeve defines control sections for engaging and disengaging the clutch mechanism and for locking and unlocking the four sun gears. Each sun gear has sun pawls engageable with projections formed on the fixed shaft. The control sections of the shifter sleeve cause the sun pawls to engage the projections, thereby to lock the sun gears, or to mount the projections, thereby to unlock and allow rotation of the sun gears. In this way, the self-contained speed change apparatus transmits torque from the drive member to the hub body in seven, a first to a seventh, speed stages.
A relatively strong operating force is required to switch the sun pawls from a lock position to an unlock position since the sun pawls must be disengaged from the projections on the fixed shaft. A strong shifting force is required particularly when a large driving load is applied during a run, for example, to maintain the sun pawls rigidly engaged with the projections on the fixed shaft. In conventional practice, therefore, a relatively strong return spring is used to realize a smooth shifting operation based on the force of the return spring against the rigid engagement between the sun pawls and the projections on the fixed shaft. However, a shifting operation carded out by pulling the control cable requires a strong force to overcome the strong return spring.